Harvesters (e.g. combines) are used to harvest crops. Operations performed by these combines include threshing and separating grain from crop residue. Ideally, only grain falls through perforated pans into a cleaning system to be collected. However, unwanted residue also falls through the pans and into the cleaning system. A fan is employed in the cleaning system in order to remove this residue. The goal is to blow the residue out of the cleaning system while retaining the grain. Conventional combines, however, have no means of automatically adjusting the fan speed during harvesting. Rather, the fan speed is set at a constant level during harvesting. When throughput is high, the fan speed may be too low to properly blow the residue out of the cleaning system, resulting in an unacceptable amount of residue being collected with the grain. When throughput is low, the fan speed may be too high resulting in the grain being blown off of the cleaning system, thereby increasing loss.